1. Field of the Invention
The present invention relates to a radio frequency coil for magnetic resonance imaging or spectroscopy, and more particularly, to a radio frequency volume coil design having a biplanar geometry in which two sets of conductors, each carrying currents in opposite directions, lie on the surfaces of two parallel planes.
2. Description of the Prior Art
In magnetic resonance imaging (MRI) and spectroscopy, it is desired to develop a highly uniform excitation profile (magnetic field) across the object under investigation. Many recent designs for radio frequency (RF) coils have been based on the paradigm that a cylindrical sheet of current density will produce a perfectly uniform transverse component of magnetic field so long as the current density, J, is distributed according to the cosine of the azimuthal angle, .theta., in accordance with the following equation: ##EQU1## where .mu..sub.0 is the permeability of free space, r is the radius of the coil, B.sub.1 is the measured magnetic field, and k is a unit vector parallel to the axis of the cylinder. Hence, the so-called "birdcage" coil developed by Hayes et al. and described in an article entitled "An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body NMR Imaging at 1.5 T," Journal of Magnetic Resonance, Vol. 63, p.622 (1985) has been widely used. A birdcage coil approximates the current distribution of Equation (1) by placing a number of wires at equal spacing around a cylinder. The desired resonant mode of the birdcage coil is thus one in which a cos (.theta.) current distribution exists in the wires of the coil. A birdcage design is advantageous in that it offers the advantage of being usable in a quadrature mode, which decreases the power requirements by a factor of 2 and increases the sensitivity by a factor of .sqroot.2. In an alternative design, proposed by Bolinger et al. in an article entitled "A Multiple-Frequency Coil With a Highly Uniform B.sub.1 Field," Journal of Magnetic Resonance, Vol. 81, p.162 (1988), a number of wires, each carrying the same current, is placed around a cylinder to form a discrete cosine coil in which the spacing between the wires is proportional to cos (.theta.).
Although cylindrical coils have been shown to provide homogeneous magnetic fields, cylindrical coils typically require many wires and are thus relatively difficult to build and, as a result, are quite expensive. In addition, as described by Schnall et al. in an article entitled "A Technique for Simultaneous .sup.1 H and .sup.31 P NMR at 2.2 T in Vivo," Journal of Magnetic Resonance, Vol. 63, pp.401-405 (1985), coil designs such as the birdcage design are quite difficult to tune to multiple frequencies. Moreover, sensitivity of such coils is limited because of the difficulty of placing cylindrical coils in close proximity of the object being investigated. Furthermore, and quite importantly, it is often quite difficult to optimally place a patient within a cylindrical coil since the coil is only open at its ends. In other words, for imaging a patient's torso and the like, the diameter of the coil must be increased to accommodate the patient's shoulders, which leads to reduced sensitivity of the coil. Also, since the patient must slide into an end of the cylindrical coil for upper body measurement, the patient must be completely surrounded by the cylindrical coil, which can cause acute claustrophobia.
Accordingly, it is desired to develop an alternative coil design which increases the sensitivity of the measurements in order to increase the achievable resolution of the resulting images while also providing for easy access for upper body imaging of a patient, for example, at a reduced cost. In particular, the present inventors feel that given the extremely high field homogeneity of coil designs presently in use, such as the birdcage coil, that there is room for a further tradeoff of homogeneity for sensitivity. The biplanar RF coils of the invention have been designed to meet these needs.